Performing fluid flow measurements for LCG, such as liquid petroleum gas (LPG), entails a broader set of challenges that are not present in the measurement of other fluids. For the product to remain in a liquid state, LCG require the pressure of the fluid system to be maintained above the vapor pressure (i.e., the pressure at which a liquid-gas equilibrium occurs) for the fluid. If the pressure in the fluid system drops below the characteristic vapor pressure of the product, the liquid flashes (evaporates) to its vapor or gaseous state.
The presence of a mixture of a vapor and a liquid in certain mass flow meters, such as Coriolis mass flow meters, can detrimentally affect the accuracy of the mass flow measurements and other measurements. For example, the mixture of gas and liquid in the fluid flowing in a Coriolis mass flow meter causes a decrease in the amplitude of vibration and a corresponding decrease in measurement accuracy. Error in density measurements is another detrimental effect of the presence of gas in fluid flow measurements. Because the average density of a liquid is greater than the average density of a gas, the presence of a mixture of gas and liquid in the fluid will yield an average density measurement that is too heavy for a gas and too light for a liquid. If such density measurement is used to convert measured mass to volume, the calculated volume could have a significant error when compared to calibrated volumetric references.
Existing flow measurement systems rely on ancillary devices, such as vapor eliminator tanks, to provide vapor reduction. Vapor eliminator tanks can be used to trap some of the vapor originating before the eliminator tanks, but do not typically control vapor forming after the eliminator due to critical pressure drops. Another approach uses differential pressure valves to control line pressure, but the settings on the differential pressure valves are limited to a particular product with a particular vapor pressure. Therefore, such approach relies on the concept that the product's vapor pressure characteristics will remain unchanged. Other technologies use pressure transducers, control valves, and programmable logic controllers to control the line pressure. Such can be a technically effective alternative, but requires expensive equipment and extensive support.
U.S. Pat. No. 6,471,487 to Keilty, et al. discloses a fluid delivery system that includes a Coriolis mass flow meter, a pump, a recirculation valve, and/or a back pressure valve. The fluid delivery system prevents the measurement of a multiphase fluid flow without the need for an air eliminator and strainer. If the measured density value exceeds one or more comparison values, the flow meter automatically shuts down the pump and closes the back pressure valve to stop the delivery of the fluid product from the fluid source to the destination to prevent the measurement of a multiphase fluid flow. However, this method requires extra piping for the recirculation path.
Other examples of U.S. patents relating to Coriolis flow meter technology include U.S. Pat. No. 7,114,517 to Sund, et al., U.S. Pat. No. 5,927,321 to Bergamini, and U.S. Pat. No. 5,804,741 to Freeman.
The disclosures of the foregoing patents are fully incorporated herein for all purposes.
There is a need for a fluid flow measurement system and methodology that reduces the presence of vapor using a minimal amount of equipment and minimal cost. While various methodologies have been developed for reducing detrimental effects caused by the presence of vapor in fluid flow measurement systems, no design has emerged that generally encompasses all of the desired characteristics, in an adaptive and dynamic form, as hereafter presented in accordance with the subject technology.